Insulin receptors have been demonstrated on various human tissues and animal tissues. This study utilizes the insulin receptor associated with cultured mouse macrophages (P388D1) and macrophage-containing cell population from normal mouse spleen. Insulin, at physiological concentrations, inhibits the ability of these cell populations to lyse antibody-coated red blood cells. Thus, these cell populations can be used to measure receptor function by following a biological response. BIBLIOGRAPHIC REFERENCE: Bar, R.S., Kahn, C.R., and Koren, H.S.: Insulin inhibition of antibody-dependent cytotoxicity and insulin receptors in macrophages. Nature 265: 632-635, 1977.